Most of fuel oil tanks in gas stations or the like have heretofore been buried underground. In the underground tank, micro-cracks are generated in a short time by degradation with the lapse of time, and there is a very strong possibility that oil leakage occurs. When the tank reaches this situation, ambient pollution is caused, and recovery requires enormous expenses. Therefore, in the underground fuel oil tank, presence/absence of oil leakage (or tank cracks which cause the leakage) is obliged to be periodically detected.
As a method which has heretofore been used for this oil leakage detection, there is a method in which gases such as air are pressurized and injected into the tank in a sealed state of the tank, and presence/absence of a pressure decrease after the lapse of a predetermined time is detected. Conversely, there is a method in which a pressure in the tank is reduced in the sealed state of the tank, and the presence/absence of a pressure increase after the lapse of the predetermined time is detected. However, in these methods, an operation for sealing all openings of the tank with putty or the like is required prior to a leakage detection operation, an operation for stopping the use of the tank to remove all oil from the tank is required as the case may be, and the operation becomes very troublesome. Additionally, when the tank is not completely sealed, the leakage detected in these methods does not necessarily reflect actual oil leakage based on tank cracks or the like, and detection precision is not so high for a trouble of the detection operation.
On the other hand, as another method of liquid leakage detection, as described, for example, in JP-A-62-223640 and JP-A-10-120099, there is a method in which a fluctuation of a liquid level is detected. In this method, the fluctuation of the liquid level is measured based on a volume change of the liquid in the tank by the leakage, and therefore detection is possible while the leakage is accurately reflected. However, in this method, when a leakage amount is small, the fluctuation of the liquid level is remarkably small, and it is therefore remarkably difficult to detect the fluctuation.
To quickly cope with the leakage of the liquid in the tank, it is important to detect the leakage in an early stage in which the cracks of the tank are small and the leakage is little. Therefore, when the detection of a small amount of leakage is demanded, the method of detecting the fluctuation of the liquid level can not sufficiently meet the demand.
Therefore, an object of the present invention is to provide a device for detecting leakage of liquid in a tank, which is capable of detecting even a small amount of leakage easily and accurately.
Moreover, an object of the present invention is to provide a device for detecting leakage of liquid in a tank, which is capable of detecting leakage without stopping the use of the tank.
Furthermore, an object of the present invention is to provide a device for detecting leakage of liquid in a tank, which can be attached to the existing tank without any special working.
Additionally, an object of the present invention is to provide a device for detecting leakage of liquid in a tank, which is capable of grasping a leakage amount accurately.